This invention relates generally to electromagnetic devices for erasing recorded information from magnetic recording tape, and, more particularly, to such devices of the type operated by direct current (dc).
Typically, tape recording systems include an erase head mounted in a leading position with respect to a recording transducer head, so that previously recorded material on the tape will be erased immediately before new material is recorded. It is well known that magnetic materials, including those used to coat magnetic recording tapes, exhibit a property known as hysteresis, wherein they will retain a degree of residual magnetism after a magnetizing or magnetomotive force is removed. Accordingly, if a constant magnetic field were to be applied in a direction generally parallel to the direction of movement of the tape, each particle of magnetic material on the tape would be subjected to a gradually increasing magnetic field strength as it approached the constant field, and to a gradually decreasing magnetic field strength as it moved away from the field. However, because of the effect of hysteresis, each such particle of magnetic material would remain partially magnetized in the direction of the constant magnetic field.
The aforedescribed asymmetry in the direction of magnetization of a tape erased by a constant magnetic field is of little consequence in some magnetic recording applications, such as digital recording, but is highly significant in the recording of analog signals, as in video tape recorders. In analog recording, the asymmetry can give rise to distortion in the recorded and subsequently played back video signal. This distortion manifests itself by the presence of unwanted second harmonic components, and is totally unacceptable for video applications.
For the foregoing reason, video and audio tape recorders typically employ high-frequency, alternating-current (ac) erase heads. As each particle of magnetic material approaches an ac erase head, it is subjected to rapid magnetization in alternating directions. Because of the hystersis effect, the degree to which the particle is magnetized lags behind the cyclic variation in the magnetizing force, and the relationship between magnetic field strength and magnetizing force follows a well known characteristic hystersis loop. As the particle moves away from the ac erase head, the magnitude of the magnetizing force gradually diminishes, and the hystersis loop becomes progressively smaller and smaller, until, when the particle has escaped the influence of the field, it remains in an essentially neutral magnetic condition, without any asymmetrical magnetic bias in one direction or another.
Although ac erase heads are entirely satisfactory for most purposes, they have the disadvantage of relatively high cost as compared with a comparable dc erase head. Furthermore, an ac erase head must be provided with power from a high-frequency power source, and there is a significant problem in dealing with unwanted radiation from the source. Accordingly, appropriate shielding has to be provided, at a correspondingly higher cost.
It will be appreciated from the foregoing that there is still a significant need for a practical alternative to the more conventional ac erase head for magnetic tape recording, and such an alternative should not have the inherent disadvantage of asymmetrical magnetization, usually associated with dc erase heads, and should still be capable of erasing a single, relatively narrow recording track on a multi-track tape. The present invention fulfills this need.